Today medical personnel are able to monitor and diagnose a patient's physical condition by noninvasive techniques using various types of monitoring and diagnostic equipment. For example, an attending physician may determine the cardiac activity of a patient by reviewing an electrocardiogram (ECG) produced by ECG monitoring equipment connected to the patient. Additional diagnostic equipment may further analyze the ECG signals and permit the physician to make a thorough diagnosis of the patient's cardiac system.
Medical monitoring and diagnostic equipment typically receive patient physiological signals from electrode leads that connect the medical equipment to the patient. The electrode leads usually consist of an electrode attached to the patient and an electrical conductor that connects the electrode to the medical equipment. In order to protect the patient from being inadvertently shocked by the medical equipment, the electrode leads and, thus, the patient, are isolated from ground. By isolating the patient from ground, the patient is protected from potentially harmful ground currents associated with the nonisolated medical equipment.
The electrode leads are typically isolated from ground by connecting them to the medical equipment through isolating couplers, such as inductive or optical isolating couplers. Both inductive and optical isolating couplers are well known in the prior art and both present problems that are solved by the apparatus of the present invention. Basically, an inductive coupler is a transformer. One winding of the transformer is connected to the patient lead and the other winding is connected to the medical equipment. The mutual inductance between the transformer windings permits the patient physiological signals to be inductively (i.e., magnetically) coupled to the medical equipment. These inductive couplers are normally housed in the medical equipment. Unfortunately, the inductive couplers are large and bulky in comparison to the other electronic components used in the medical equipment. As a result, efforts to reduce the physical size of the medical equipment may be limited by the relatively large size of the inductive couplers. Additionally, electrical noise produced by other electrical components in the medical equipment may be inductively coupled through the windings of the transformer and shock the patient or corrupt the patient's physiological signal. Furthermore, inductive couplers are relatively expensive components that increase the cost of manufacturing the associated piece of medical equipment.
Optical couplers do not experience many of the drawbacks associated with inductive couplers. Optical couplers are typically smaller and less expensive than inductive couplers. Also, optical couplers are not affected by electrical noise present in the medical equipment. For these and other reasons, optical couplers are generally preferred over inductive couplers; however, as discussed below, optical couplers suffer other drawbacks.
A common type of optical coupler uses a light emitting diode (LED) and a photodetector. The LED is connected to the patient leads and the photodetector is connected to the medical equipment. The presence of a patient physiological signal causes the LED to emit a beam of light, which is detected by the photodector and transmitted to the medical equipment as an electric signal. Patient physiological signals may be applied to optical couplers in either analog or digital form. When the physiological signals are applied in analog form, the intensity of the emitted beam of light varies with the amplitude of the analog signal. The photodetector detects the emitted beam of light and produces an electrical signal whose amplitude is proportional to the intensity of the emitted beam of light. Unfortunately, the LEDs do not operate linearly over a broad range of physiological signal amplitudes. As a result, the magnitude of the electric signal transmitted to the medical equipment may not accurately represent the amplitude of the patient physiological signal.
The nonlinearity problem associated with optical couplers may be avoided by converting the analog physiological signal to a digital signal. However, this requires a complex analog-to-digital conversion of the patient physiological signal, which increases the cost and complexity of the associated medical equipment.
As can be readily appreciated from the foregoing discussion, there has developed a need in the medical profession for an apparatus that will permit electrode leads to be optically coupled to medical monitoring and diagnostic equipment while avoiding the nonlinearity and complex analog-to-digital conversion problems associated with optically coupled medical equipment in the prior art. The present invention provides an apparatus for transmitting patient physiological signals that achieves these results.